fix: keep a generator- inside a package or generator name in the namespace

[mshima]

Summary

asNamespace removed everything up to the last generator- of the path, wherever it was:

Path	Before	After
generator-foo-generator-bar/generators/app/index.js	bar:app	foo-generator-bar:app
@scope/generator-foo-generator-bar/generators/app/index.js	@scope/bar:app	@scope/foo-generator-bar:app
generator-foo/generators/my-generator-thing/index.js	thing	foo:my-generator-thing
@scope/plain/generators/app/index.js	@scope/@scope:plain:app	throws

• Only the last folder starting with generator- is the package: the path up to it and its generator- prefix are removed, a generator- inside a package or generator name is kept. The existing cases (absolute paths, node_modules, Windows paths, generator-backbone/generator-backbone/all.js) are unchanged.
• A path in a scoped package that is not a generator- one has no namespace: asNamespace (and Environment#namespace) throws instead of returning a broken one.
• Store#lookupSync computes the namespace inside its try: such a generator is reported as not registered (and logged) and the lookup goes on with the other generators.

Behavior change

Packages with generator- inside their name (generator-foo-generator-bar) are now registered under their full name (foo-generator-bar), like namespaceFromPackageName gives it.

Test plan

• eslint, tsc
• esmocha: tests for generator- inside package and generator names, the throw for scoped non generator- packages, and a lookup going on after such a package

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